This invention relates to a device for controlling exhaust gas recirculation (EGR), particularly as applied to motor vehicles driven by internal combustion engines.
The circulation of portions of internal combustion engine exhaust back to the engine combustion chambers is coming into general use for suppressing to some extent the formation of NOx in the engine exhaust, the idea being to introduce inert substances, i.e., combusted exhaust gas into the combustion chamber in order to lower peak combustion temperatures therein, thereby reducing NOx formation. An EGR valve is used to control recirculation of the exhaust gas. The valve is typically a vacuum operated valve.
This invention is particularly directed to "amplifier" EGR systems, as opposed to "ported" EGR systems. Amplifier system are distinguished from ported systems in that the former utilize a vacuum amplifier controlled by venturi vacuum for providing controlled modulated vacuum to an EGR valve. In the ported systems the vacuum to the EGR valve is modulated by the throttle valve at a port in the engine carburetor bore. Amplifier EGR system has heretofore tended to have limited capacility for controlling NOx emissions. Specifically, manifold vacuum tends to vary widely depending on engine operation thus effecting the control of the EGR valve. Even if the level of the venturi vacuum is constant or the amount of induction air is constant, the amount of the exhaust gas recirculated to combustion chambers changes with respect to manifold vacuum, in other words, the amount of same is considerably larger when the manifold vacuum is at a considerably high level, for example, during low engine loads or decelerations as compared with when the manifold vacuum is at a considerably low level, for example, during high engine load operations. This variation of the amount of recirculated gas with respect to the amount of induction air invites unstable engine operations.